Multi-host cross-compile build sometimes fails when building llvmdeps.rs

[brson]

This happens on our nightly and dist bots periodically:

make[2]: Nothing to be done for `all'.
make[2]: Leaving directory `/home/rustbuild/src/rust-buildbot/slave/dist2-linux/build/obj/i686-unknown-linux-gnu/llvm/bindings'
llvm[1]: ***** Completed Release+Asserts Build
make[1]: Leaving directory `/home/rustbuild/src/rust-buildbot/slave/dist2-linux/build/obj/i686-unknown-linux-gnu/llvm'
Traceback (most recent call last):
  File "/home/rustbuild/src/rust-buildbot/slave/dist2-linux/build/src/etc/mklldeps.py", line 50, in <module>
    out = run([llconfig, '--host-target'])
  File "/home/rustbuild/src/rust-buildbot/slave/dist2-linux/build/src/etc/mklldeps.py", line 38, in run
    proc = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
  File "/home/rustbuild/root64/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/home/rustbuild/root64/lib/python2.7/subprocess.py", line 1327, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory
make: *** [/home/rustbuild/src/rust-buildbot/slave/dist2-linux/build/src/librustc_llvm/llvmdeps.rs] Error 1
program finished with exit code 2
elapsedTime=4174.547997

Here's what configure looks like:

../configure --disable-valgrind --enable-dist-host-only --host=i686-unknown-linux-gnu,x86_64-unknown-linux-gnu

[brson]

Presumably it's complaining that llvm-config can't be found, but the makefiles look right to me. Hard to reproduce.

[alexcrichton]

I've only ever seen this get reproduced during an LLVM upgrade on a multi-host builder. I've only sporadically been able to reproduce locally, and if you re-make after the error occurs it does not repeat itself. The only sequence of events I've seen to trigger this is:

1. Build LLVM in the rust repo (for multiple hosts).
2. Upgrade LLVM in the rust repo.
3. Build LLVM again (run make -j), see the error.

[brson]

Perhaps this is because the rules for the host-specific llvm stamp call the non-host-specific clean-llvm target, so when the build gets to the second LLVM it deletes the just-built LLVM.